Generally, as an electrostatic coating apparatus, for example, there have been known paint coating apparatus which are constructed of an atomizer consisting of an air motor and a rotary atomizing head, a housing member adapted to hold the air motor of the atomizer, and a high voltage generator adapted to impart a negative high voltage electrostatic charge to sprayed paint particles (e.g., see, for example, Japanese Patent Laid-Open No. H10-57848 and Japanese Utility Model Laid-Open No. H3-75856).
In the case of prior art electrostatic coating apparatuses of the sort mentioned above, an electrostatic field is formed by lines of electric force between a rotary atomizing head which apply a negative high voltage electrostatic charge and a work piece. In this state, if the rotary atomizing head is put in high speed rotation to spray paint, sprayed paint particles which have been sprayed from a rotary atomizing head are electrostatically charged to become charged paint particles which are imparted with a negative high voltage electrostatic charge. Thus, charged paint particles are urged to fly toward and deposit on surfaces of a work piece which is connected to the earth.
Further, according to the prior art, repulsive electrodes are located on an outer peripheral side of a housing member, and a high voltage of the same polarity as charged paint particles is applied to the repulsive electrodes. By a repulsion force which occurs between a repulsive electrode and charged paint particles, the latter are urged to fly toward a work piece and prevented from depositing on the housing member.
In the case of electrostatic coating apparatuses disclosed in above-mentioned Japanese Patent Laid-Open No. H10-57848 and Japanese Utility Model Laid-Open No. H3-75856, a repulsive electrode which is located around the outer periphery of a housing member has a repulsion force effective against charged paint particles but not effective enough against electrostatically attenuated paint particles floating around a housing member and carrying only an attenuated quantity of electrostatic charge.
Further, in the case of electrostatic coating apparatus in Japanese Patent Laid-Open No. H10-57848 and Japanese Utility Model Laid-Open No. H3-75856, in order to prevent spark discharges between a repulsive electrode and a grounded body, the repulsive electrode is formed in the shape of a ring or ball with smooth surfaces to avoid concentration of electric field. Therefore, the repulsive electrode is incapable of supplying a sufficient quantity of discharge ions to outer surfaces of a housing member, resulting in failure in maintaining a high voltage electrostatic potential on outer surfaces of the housing member.
As a consequence, as an electrostatic coating operation is continued, paint particles start to gradually deposit on outer surfaces of the housing member and remain there as paint deposits. Such paint deposits give rise to a problem of degradations in insulation quality of outer surfaces of the housing member.
On the other hand, in order to maintain a housing member at a high voltage electrostatic potential, for example, there may be employed a repulsive electrode of a larger diameter to broaden high voltage discharge surface areas. However, in this case, there is a necessity for keeping the repulsive electrode at a sufficient distance from a work piece to prevent spark discharges which might occur between the repulsive electrode and a work piece or other grounded body since a high voltage is constantly applied to the repulsive electrode.
Therefore, in addition to degradations in performance quality, an operative range of an atomizer is narrowed by the use of a repulsive electrode of a large outside diameter. Especially, in a case where a coating operation is carried out in a narrow space like inside of vehicle body, difficulties are encountered in keeping a distance between a repulsive electrode and a grounded body like a vehicle body, and a coating operation becomes infeasible.